An exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions and control noise. The exhaust system includes an injection system that injects a diesel exhaust fluid (DEF) or a reducing agent, such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst. A mixer is positioned upstream of the SCR catalyst and mixes engine exhaust gases and products of urea transformation.
The doser typically sprays the urea into the exhaust stream. In one known configuration, the doser is mounted to an outer peripheral surface of the mixer and directs the spray radially inwardly toward a center of the mixer. The spray tends to have a larger droplet size in order to provide a high spray penetration such that the droplets are spread across the entire exhaust cross-section. However, smaller droplet sizes are preferred as droplets with smaller diameters evaporate more quickly than larger diameter droplets. Due to packaging constraints it is important to provide a compact configuration for the mixer by minimizing the length of the mixer. Typically, mixers having a short length also have a large exhaust flow cross-sectional area. This is disadvantageous when using smaller droplet sizes as the droplets tend not to penetrate sufficiently into the exhaust flow. Thus, having the desired shorter mixing length and the desired smaller spray droplet size are at odds with each other. Not only must the droplets have a high spray penetration, the droplets must be distributed evenly.